Reactive production techniques for ceramic or cerametallic parts, for the surfaces of functional parts or of structural members and for the matrices of composites reinforced with fibers or other reinforcing elements are among the particularly uncomplicated and economical production methods within the field of materials technology. The principle is based on the production of high-value added materials by way of a reaction between two or more raw materials--often inexpensive ones--at temperatures which are very often lower than the usual (reaction-free) production temperatures. Typical features of reactive production methods are: low cost, near net shape and high purity of the product.
New strengthening strategies for ceramic materials are based on incorporating a second phase in the ceramic matrix, with the useful properties of the ceramic material being for the most part retained. Examples here are transformable ZrO.sub.2 particles ("Strengthening Strategies for ZrO.sub.2 --toughened Ceramics at High Temperatures", J. Mat. Sci. Eng., 71 (1985) 23) or SiC whiskers ("TZP Reinforced with SiC Whiskers", J. Am. Ceram. Soc., 69 (1986) 288) in an Al.sub.2 O.sub.3 matrix. The incorporation of metals was initially considered not to be useful, since according to traditional theories on composites, metals, with low yield limits and low elastic moduli, would not be able to improve hard and rigid ceramics, especially not in respect of their strength. Admittedly, it has been shown recently that this is not always the case ("Effect of Microstructure on Thermal Shock Resistance of Metal-Reinforced Ceramics", J. Am. Ceram. Soc. 77 (1994) 701 and "Metalle verbessern mechanische Eigenschaften von Keramiken", Spektrum der Wissenschaft, Januar (1993) 107). However, the strengthening effect is only achieved if the microstructure of the classical cermets is reversed, i.e. the ceramic material forms a rigid matrix which is permeated by a monocrystalline metallic phase. The designation "metcers" would in this case constitute a conceptual characterization of the reversal in microstructural components. Quite apart from the modified microstructure of these composites, it is also the significantly smaller quantity of the metal phase which is responsible for the improvement compared to conventional cermets. The metal embedded in the ceramic matrix has considerably better mechanical properties than it does in the "free" state, a phenomenon which seems to apply even for otherwise brittle intermetallic phases ("Metcers--a Strong Variant of Cermets", Cfi/Ber. DKG 71 (1994 301).
So far, a number of different methods have been used to produce these novel metal-ceramic composites, for example the directed oxidation of molten metals (DMO), where an Al/Al.sub.2 O.sub.3 composite grows on molten aluminium by way of oxidation in air (see e.g. "Formation of Lanxide.TM. Ceramic Composite Materials", J. Mat. Res. 1 (1986) 81 and "Directed Oxidation of Molten Metals" in: Encyclopedia of Mat. and Eng. (Ed. R. W. Cahn), Supplementary Vol. 2, Pergamon, Oxford (1990) 1111). Other practicable methods are pressure-die casting ("Application of the Infiltration Technique to the Manufacture of Cermets", Rep. Dt. Keram. Ges., 48 (1971) 262-8) and the infiltration of porous ceramic preforms with molten metal ("Method for Processing Metal-Reinforced Ceramic Composites", J. Am Ceram. Soc., 73 [2] 388-393 (1990). Gas-pressure infiltration provides a means of infiltrating non-wetting metals into the ceramic preform (see e.g. "Microstructure and Properties of Metal Infiltrated RBSN Composites", J. Eur. Ceram. Soc. 9 (199161-65). The metal is first melted in a vacuum and then, once the infiltration temperature has been reached--usually 100 to 200.degree. C. above the melting point--the ceramic preform is dipped into the molten metal and a gas pressure built up. This technique is also suitable for metals with high melting points, which cannot be infiltrated into the ceramic preform using the conventional pressure-die casting method, but is time-consuming and very expensive.
Another method of producing an Al.sub.2 O.sub.3 part permeated by Al is based on reactive metal infiltration of ceramic preforms containing SiO.sub.2 (see e.g. Al.sub.2 O.sub.3 /Al Co--Continuous Ceramic Composite (C.sup.4) Materials Produced by Solid/Liquid Displacement Reactions: Processing, Kinetics and Microstructures", Ceram. Eng. Sci. Proc. 15 (1994) 104).
Composite parts containing Al and Al.sub.2 O.sub.3 can also be produced by means of thermite-based reactions (SHS: Self-Propagating High-Temperature Synthesis). A large number of such reactions have been investigated so far, all of which proceed according to the scheme: EQU MO+Al.fwdarw.Al.sub.2 O.sub.3 +M,
where M is a metal and MO the corresponding oxide (see e.g. "Combustion Synthesis of Ceramic and Metal-Matrix Composites", J. Mat. Synth. Proc. 2 (1994) 71 and "Thermodynamic Analysis of Thermite-Based Reactions for Synthesis of Oxide-B.sub.4 C Composites", J. Mat. Synth. Proc., 2 (1944) 217 and 227). As a result of the uncontrollable generation of heat (the reaction is highly exothermic) all SHS composites are porous, inhomogeneous and of coarse microstructure. As a result their strength seldom exceeds 100 MPa, which means their use as structural members is out of the question.
Research in the field of materials has for a long time pursued the goal of substituting intermetallic compounds for metals in many areas, also in metal-ceramic composites. The intermetallic compounds of Al (aluminides) are especially in demand here due to their low specific gravity, good high-temperature stability and their resistance to oxidation (see e.g. "Intermetallic Compounds", Mat. Res. Soc. Proc. Vol. 288, 1993). However, the powder-metallurgical production of aluminides with ceramic phases has up till now been very costly, since, on the one hand, the production of the aluminide powder is very expensive because of the extremely inert conditions required and, on the other hand, the powder can only be fully compacted by means of hot pressing, hot forging, hot extrusion, hot isostatic pressing, or explosive forming (see e.g. "Powder Processing of Intermetallics and Intermetallic Matrix Composites (IMC)" p. 93-124 in Processing and Fabrication of Advanced Materials for High-Temperature Applications-II, ed. V. A. Ravi et al, The Min. Met. Mat. Soc., 1993). Moreover, in all cases the aluminide constitutes the matrix, while the Al.sub.2 O.sub.3 is dispersed as particulate phase and makes up less than 50% of the volume (see e.g. "A Review of Recent Developments in Fe.sub.3 Al--Based Alloys", J. Mat. Res. 6 (1991) 1779 and "Powder Processing of High-Temperature Aluminide-Matrix Composites", H-T Ordered Intermetallic Alloys III, 133 (1988) 403). For the production of such composites use can be made of the reaction heat by having two or more metals react with each other to form the desired aluminide, but in all cases investigated so far this procedure results in coarse and inhomogeneous microstructures, which means that mechanical properties were either not measured at all ("Reactive Sintering Nickel-Aluminide to Near Full Density", PMI 20 (1988) 25), or that, in a further step, the preformed part had to be subjected to hot-wet compaction ("SHS of TiAl--SiC and TiAl--Al.sub.2 O.sub.3 Intermetallic Composites", J. Mater. Sci. Let., 9 (1990) 432).
All hitherto known forms of composites and the methods of production thereof have characteristic disadvantages. The pressure-die casting technique, for example, is for technical reasons (no suitable pressure-vessel material available) only suitable for Al alloys, not, however, for high-melting-point aluminides. Similar limitations apply to gas-pressure infiltration, where aluminides can only be infiltrated at temperatures far in excess of 1400.degree. C. Moreover, in this case the infiltrated composite part would still have to be extricated from the solidified aluminide melt, an extremely tedious procedure and only possible for parts with simple geometry. Reactive-type processes such as DMO and C.sup.4 can only be used for Al.sub.2 O.sub.3 parts with Al alloys containing Si or Mg, i.e. not for alloys containing aluminides. Besides, the reaction speeds are extremely low, averaging only 2 cm/day, which means that the procedure is extremely time-consuming. All powder-metallurgical processes used hitherto have resulted in the disadvantages typical of oxide-ceramic cermets, i.e. without subsequent hot compaction, the microstructure is porous, coarse (microstructural components usually being much larger than 10 .mu.m) and inhomogeneous, which results in inadequate strength as well as brittleness.
According to a process suggested at an earlier date, in which a number of the aforementioned disadvantages are eliminated, a green compact is formed out of an intensively mixed and ground powder mixture comprising Al, oxides and, if required, further additives; during heat treatment in an inert atmosphere the green compact is converted, i.e. reaction-formed (3A process) into a composite containing Al.sub.2 O.sub.3 and aluminide. This process has the following disadvantages: a) heat treatment has to be carried out at temperaturtes &gt;1400.degree. C., b) reaction and compaction incur shrinkage of between 10 and 20% which, among other things, makes it difficult to apply the process to composites, c) the intensive grinding has to be performed in organic solvents, making the process expensive and not particularly environmentally sound, d) the finely-ground aluminium fraction renders the mixture highly inflammable, necessitating expensive safety measures, e) the proportion of aluminium which oxidizes during grinding (necessary for safety reasons) cannot be adjusted accurately, f) the process is more suitable for solid parts and less for coatings.
The object of the present invention is thus to avoid the disadvantages of the above-mentioned process without causing any deterioration in the product.
The various features of novelty which characterizes the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects obtained by its use, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated and described a preferred embodiment of the invention .